| With the increasing growth of primary energy consumption, high-quality coal reserves have dropped significantly. Mining and use of low-rank coal, especially for lignite resources, is a reasonable way to relieve stress for energy demand. Lignite reserve in China is relatively abundant, but the high moisture content, low calorific value and easy moisture re-adsorption greatly restrict its utilization fields. The abundant polar oxygen-containing groups in lignite with the great hydrophilia are the main factors leading to its high moisture content and the big adsorbing moisture capacity of dewatered sample. Therefore, researching the influence of oxygen functional groups on the moisture adsorption is very important for the clean and efficient use of lignite.In this paper, SL lignite was selected as a research object. It was treated with alkali solution and thermal upgrading process. Chemical titration method was used to determine the content of oxygen-containing groups in lignite before and after treatment, and the relationship between oxygen-containing groups and moisture re-adsorption of lignite was also investigated. The main findings include the following aspects:(1) There exist many kinds of acidic oxygen-containing groups in lignite. Phenolic hydroxyl accounts for mainly, carboxyl in which some of carboxyl groups is carboxylate, and the carbonyl exists for a certain proportion. Alkaline solution can effectively change the content of acidic oxygen-containing groups in lignite. The acidic oxygen-containing groups dissociates when the lignite is treated with alkaline solution, and it does more greatly with the increased pH value of alkaline solution. The hydrophilicity of lignite weakens after alkaline solution treatment due to the decrease of oxygen-containing groups, which results in a slow adsorption rate and reduction of adsorbing moisture amount.(2) The different thermal upgrading conditions lead to various changes of chemical structure for lignite. The weight loss rate of lignite increases with rising temperature, but along with varied change trends of different containing-oxygen groups in the whole temperature region. Carboxyl is the most susceptive group to temperature, which can be decomposed dramatically at above 160℃. Carbonyl groups start to breakdown at above 200℃, while carboxylate and phenolic hydroxyl groups appear to be stable up to 240℃. Drying medium also greatly affects the removal of oxygen-containing groups, which proceeds more easily in CO2 atmosphere compared with that in Ar. The oxygen-containing groups increase first because of oxidation reaction of lignite exposed to air in the upgrading process and then decrease due to the decomposition.(3) The hydrophilia of lignite is mainly controlled by pore structure and oxygen-containing groups. The adsorbing moisture capacity of dewatered lignite changes mainly with the surface area at low drying temperature and the contributions of oxygen-containing groups are obvious. The relationship between moisture re-adsorption content of q (mg/g) and oxygen-containing groups of n (mmol/g) accords with following formula:q= 55.19+15.83n.cooH +9.90nAr-oH+9.65n_c=o. Carboxyl, as the polar oxygen group, contributes the most to the moisture adsorption content.(4) The isosteric heat of adsorption is relevant to the adsorbing moisture content of upgraded lignite. As the upgraded lignite adsorbs less moisture, the moisture molecule combines with oxygen groups in the form of great hydrogen bond, resulting in higher isosteric heat. When more moisture is adsorbed to lignite, the weak hydrogen bond between moisture molecules leads to the decrease of isosteric heat. As the upgrading temperature rises, the isosteric heat of upgraded coal decreases. |
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